paging: pass struct osmo_mobile_identity, not encoded IE bytes

In get_paging_mi(), before this, an encoded buffer of Mobile Identity bytes is
returned. Code paths following this repeatedly decode the Mobile Identity
bytes, e.g. for logging. Also, in get_paging_mi(), since the TMSI is read in
from a different encoding than a typical Mobile Identity IE, the TMSI was
manually encoded into a typical Mobile Identity IE. This is essentially a code
dup of osmo_mobile_identity_encode(). Stop this madness.

Instead, in get_paging_mi(), return a decoded struct osmo_mobile_identity. Code
paths after this use the struct osmo_mobile_identity directly without repeated
decoding.

At the point of finally needing an encoded Mobile Identity IE (in
Encoding::write_paging_request()), do a proper osmo_mobile_identity_encode().

Since this may return errors, add an rc check for the caller of
write_paging_request(), gprs_rlcmac_paging_request().

A side effect is stricter validation of the Mobile Identity passing through the
Paging code path. Before, invalid MI might have passed through unnoticed.

Change-Id: Iad845acb0096b75dc453105c9c16b2252879b4ca
diff --git a/src/pdch.h b/src/pdch.h
index ec35174..e1e3688 100644
--- a/src/pdch.h
+++ b/src/pdch.h
@@ -48,7 +48,7 @@
 	struct gprs_rlcmac_paging *dequeue_paging();
 	struct msgb *packet_paging_request();
 
-	bool add_paging(uint8_t chan_needed, const uint8_t *mi, uint8_t mi_len);
+	bool add_paging(uint8_t chan_needed, const struct osmo_mobile_identity *mi);
 
 	void free_resources();